Introduction

Nonintrusive techniques are being extensively used in engineering measurements. These techniques employ radiation sources as probes. All radiation-based measurements share a common feature in that they generate images of a cross-section of the physical domain. This is to be contrasted with mechanical probes which are concerned with measurements at a point in space and can accomplish this task only after the field to be studied has been physically perturbed. Radiation methods are also inertia-free. Hence, the scanning of a cross-section of the physical region using radiation-based probes results in a large volume of information with practically no time delay.

Applications

Laser-based optical techniques have reached a high degree of maturity. Optical methods such as laser Doppler velocimetry have replaced traditional methods such as pilot tubes and hot-wire anemometry. FLow visualization methods of the past have evolved to a point where it is now possible to gain qualitative understanding of the flow and transport phemomena. Sophisticated measurement techniques such as Rayleigh and Mie scattering for temperature and concentration measurement and Raman spectroscopy for detection of chemical species in reacting flows are routinely employed in engineering research. Using satellite radar interferometry, orbiting instruments hundreds of kilometers away in space, can detect subtle buckling of the earth's crust and thus detect minerals and oil predict volcanic eruptions. Integrating techniques of photography and video recording, digital image processing, optics and color measurement, it is now possible to map the fluid surface slopes of oceans, rivers and lakes optically into color space. Using reconstruction techniques, the surface elevations can then be back-calculated. If the fluid surface is relatively flat, the spectrum of the reflected light contains rich information about the temperature variation over it. Radiation-based measurements from the backbone of satellite instrumentatio, weather pridiction programs and defence weaponry.

Buoyancy-driven convection is encountered in a large number of engineering applications such as cooling of electronic equipment, solar ponds, stratified fluid layers in water bodies, and materials processing to name a few.Studying convection patterns is also of importance in nuclear power plants. Specific examples include passive heat removal in advance reactor systems, stratification phenomena in steam vessels in which hot and cold water streams mix, and thermal pollution in reservoirs. The liquid metal pools in fast breeder reactors is also subjected to stratification.

Measurements of the shape of the fluid surface and temperature distribution over it are critical for studies of near surface dynamics. There has long been interest in understanding the behavior of short surface wind waves because of their importance in mass, momentum, and energy exchanges at the air-sea interface, in microwave remote sensing of the sea surface, and in the theory of wave-wave interactions. Associated problems of great interests are (a) the interaction of a free vortex with a free surface, (b) behavior of turbulence near the free surface, and (c) the effect of a variable surface tension on the shape of the free surface. A free vortex approaching the free surface may deform the surface and cause the vortex line to be connected to it. Thus, bursts originating from the lower heated wall can modify the air-water interface and significantly alter the local transport rates.

A special application where laser optics can be profitably used in growth of a crystal from its supersaturated aqueous solution. Crystals with a high degree of perfection are required for important and sensitive high-technology applications. Examples are the semiconductor industry for making computer chips and optically transparent materials for making high-power lasers. Growing methods for high-quality crystals include melt growth, flux growth and solution growth. Each of the crystal-growing processes is determinded by principles of phusico-chemical hydronamics and is extarordinarily complex. To control the process and ensure growth of large high quality crystals, it is important to understand the physical phenomena involved during crystal growth. These include all modes of heat transfer, phase change, interfacial transport, and turbulence in complex geometries. Hence, there is a need to understand the thermofluid mechanics of the crystal growth mechanism.

Measurements of the temperature and concentration fields around a crystal growing from an aqueous solution, the surface morphology, and the kinetics of major faces of the crystal is a critical engineering application. The crystal is grown in a specially-designed growth cell under controlled temperature conditions. Under growing conditions, spatio-temporal fields of temperature and concentration are setup around the crystal. The growth mechanism of the crystal, as well as its morphology, are intricately linked to temperature and concentration gradients at the crystal surface. Since these gradients setup a density field as well, the crystal will experience buoyancy-driven fluid motion around it. The solution around the crystal can be optically mapped to generate the full three-dimensional information of the scalar fields. They can be controlled online with the objective of enhancing the insitu quality of the growing crystal.